Kick up angles?

[Minisforever]

I am planning on making my own carbon chassis for my rc10. I see a lot of racers favor the 25 degree castor blocks. Which seem nonexistent compared to the 30's. So my question is has anyone tried to change the kick up to 25 degrees? Pro and cons? Thanks

[Jay Dub]

Well, technically the 25* kick up should be better as it will produce less weight jacking as the suspension moves. This is the reason the "modern" buggies favor 25* kick up. -Jeff

[Phin]

I bet most of us have changed the kick-up angle on our cars..but it's typically an increase...and done unintentionally after a hard crash or nose dive.

But seriously...the early Losi and Kyosho cars had 20-25 degree kick-ups, though there was also more involved to their suspension geometry than just the lowered kick-up by itself....e.g. the sweep on the arms also had to change in relation. I think unless you're planning to redesign the whole front end you'd probably be better off creating the chassis with their stock kick-up angles.

[Minisforever]

Thanks for the responses! So the sweep on the arm needs to be different when the kick up is changed. Say if you didn't change the sweep and went with a 25* kick what problems would accur? ie front push, chassis roll problems? I apperciate the advise. With no disrespect, I think I'm goinna go ahead and build it. It will be a "graphite" clone with a 25* kick. I have a lot of carbon cloth scraps I can use up on this little project. If it handles poorly then i will make it a shelf queen chassis.

[Phin]

When you lower the kick-up you're rotating the angle of every suspension part attached to the kick-up...and since all those parts are not centered around the same pivot point they're moving in a different arc. The further the part is from the center pivot point the more the position of that part changes.

As you lower the kick-up the castor and steering blocks move back and up which shortens the car's wheelbase and changes steering ackerman. I suspect it will also alter your roll center but I can't say for sure. Sweeping the arms forward is to correct that shift of the front axles.

[Phin]

I suppose though you could correct for the wheelbase changes by cutting your chassis a bit longer and for the ackerman change by moving the bellcranks back.

[Minisforever]

Phin, You have a lot of knowledge and great ideas thanks. That's one of the reasons I joined the forum and have not been disappointed .

I'll lay up a couple chassis in the next couple days and post up a pic. If I can figure how to post a pic.

[Jay Dub]

A five degree kick change is negligible in all other departments. The biggest change (and possibly only noticeable) change will be in castor itself. Good luck, would like to see it finished.

[Phin]

The changes seem small but I wouldn't wave it off as negligible, since they have to be considered relative to the scale of the RC. You can't just shrug off the effects of moving the steering knuckles a mm or two when changes like a couple of mm thick washers under the steering knuckle ball stud are commonly used to correct bump steer.

Also you're talking at least a half dozen things changing from just moving the kick-up. So while the changes may have been negligible by themselves, they can produce a very noticeable effect when compounded.


Granted I'll readily admit that I can't say one way or the other if all the changes that come along with lowering the kick-up will be good or bad for the car...and I apologize if I came off sounding like I don't condone experimenting. I do believe though that if someone wants to test a 25 degree kick-up then it's best to deal with all the changes that come with it together.

If however one only want to form a carbon fiber chassis with a lowered kick-up because they can't find 25 degree castor blocks then they're better off making the chassis with the stock kick-up and using B4 25 degree castor blocks. Then they can also run B4 front 2.2" wheels.

[jwscab]

you would have to use b4 20 degree caster blocks because the B4 kick up is 25 degrees, no?

[Phin]

Yeah that's right. Forgot about that.

[terry.sc]

Kickup is not that big a change, Schumacher used to use adjustable kickup on their buggies as a standard tuning option, the Cougar between 15 and 25 degrees, while the CAT 2000 could go from 7.5 to 20 degrees. The Ultima RB5 also has adjustable kickup. It has a similar effect to changing caster blocks.

Now assuming you change the caster blocks so the caster is kept the same when you change kickup, less kickup will give you more steering and the chassis will dive less under braking. It will also give more on-power steering. Increasing kickup does the opposite, but is also much better over bumps and jumps.

As for its effects on suspension geometry, as all you are doing is twisting round the lower wishbone and the upper link to match it doesn't make much difference to roll centres, although you will usually have to adjust the steering links as it will likely give bump steer as the steering arm mounting point will have moved relative to the suspension as the suspension has been twisted round.